1. Field of the Invention
The present invention relates to a temperature sensor device for detecting temperature.
2. Description of the Related Art
Conventionally, a temperature sensor device for detecting temperature, which utilizes a forward voltage of the PN junction, is well known (see, for example, Japanese Patent Application Laid-open No. Hei 5-248962).
FIG. 7 is a circuit diagram illustrating a conventional temperature sensor device. The conventional temperature sensor device includes a constant current source circuit 701 and Darlington-connected bipolar transistors 702, 703, and 704 supplied with a constant current from the constant current source circuit 701.
An emitter of the bipolar transistor 704 is connected to an output terminal. An output voltage of the temperature sensor device is the sum of forward voltages of the PN junctions of the bipolar transistors. The forward voltage of the PN junction changes in accordance with temperature, and hence the output voltage is a voltage that changes in accordance with temperature.
In the temperature sensor device configured as described above, temperature detection accuracy of the temperature sensor device is enhanced as the sensitivity with which the output voltage changes in accordance with temperature becomes higher. Therefore, the temperature detection accuracy of the temperature sensor device can be enhanced by increasing the sum of the forward voltages of the PN junctions. In general, it is known that the temperature sensitivity of the forward voltage of the PN junction is approximately 2.5 mV/° C.
In the case of the temperature sensor device of FIG. 7, the number of effective stages of PN junctions is three, and hence the sum of the forward voltages is three times the forward voltage of one PN junction. Therefore, the sensitivity with which the output voltage changes in accordance with temperature is approximately 7.5 mV/° C., which is three times the temperature sensitivity of the forward voltage of one PN junction.
In the conventional temperature sensor device, however, if the number of effective stages of PN junctions is increased for enhancing the sensitivity with which the output voltage changes in accordance with temperature, the sum of the forward voltages of the PN junctions becomes larger. Therefore, the conventional temperature sensor device has a problem that an operating voltage cannot be suppressed to be lower. This problem results in inefficiency in the sense that a low voltage range of a power supply voltage supplied from a battery or the like cannot be used.